A large number of photosensitive materials having a photosensitive layer on a support for forming images upon imagewise exposure have been known. Among them, as a system for rendering preservation of environments and image forming means simplified, a technology for forming images by heat development is exemplified.
In recent years, reduction of the amount of waste processing solutions is strongly demanded in the field of photomechanical processes from the standpoint of environmental protection and space savings. To cope with this, techniques are needed in relation to photosensitive heat developable materials for use in photomechanical processes, which can be effectively exposed by a laser scanner or laser image setter and can form clear black images having high resolution and sharpness. Such heat developable photosensitive materials can provide to customers a heat development processing system, without use of solution-type processing chemicals, simpler and free from incurring environmental destruction.
Methods for forming an image by heat development are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075 and D. Morgan and B. Shely, Imaging Processes and Materials, "Thermally Processed Silver Systems" A, 8th ed., page 2, compiled by Sturge, V. Walworth and A. Shepp, Neblette (1969). The photosensitive material used contains a light-insensitive silver source (e.g., organic silver salt) capable of reduction, a photocatalyst (e.g., silver halide) in a catalytic activity amount, and a reducing agent for silver, which are usually dispersed in an organic binder matrix. This photosensitive material is stable at room temperature. However, when it is heated at a high temperature (e.g., 80.degree. C. or higher) after the exposure, silver is produced through an oxidation-reduction reaction between the silver source (which functions as an oxidizing agent) capable of reduction and the reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image generated upon exposure. The silver produced by the reaction of the silver salt capable of reduction in the exposure region provides a black image and this presents a contrast to the non-exposure region. Thus, an image is formed.
The heat developable photosensitive materials have been known previously, but in most of those, the photosensitive layer is formed by coating a coating liquid having a solvent of an organic solvent such as toluene, methyl ethyl ketone (MEK), methanol, and the like. Use of such organic solvents as a solvent not only adversely affects human bodies during manufacturing processes but also is disadvantageous in term of costs due to recycling the solvents and others.
To cope with this, a method has been considered in which a photosensitive layer (hereinafter referred also to as "aqueous photosensitive layer") is formed using a coating liquid of a water solvent not having the above problem. For example, Japanese Unexamined Patent Publication (KOKAI) Showa (hereinafter referred to as "JP-A-") Nos. 49-52,626 and 53-116,144, and the like set forth an example that gelatin is used as a binder. Also, JP-A-50-151,138 sets forth an example that a polyvinyl alcohol is used as a binder.
In JP-A-60-28,737, an example that a gelatin and a polyvinyl alcohol are used together is described. In addition, as another example other than the above examples, JP-A-58-28,737 sets forth an example of a photosensitive layer that a water-soluble polyvinyl acetal is used as a binder.
Such a binder surely allows to form the photosensitive layer in use of a coating liquid with a water solvent, thereby making such use advantageous in terms of environments and costs.
However, if the polymer such as gelatin, polyvinyl alcohol, water-soluble polyvinyl acetal, and so on is used as the binder, the binder has a bad solubility with an organic silver salt, thereby not only rendering coatings unavailable with a surface having a practically durable quality, but also rendering a silver tone at the developed portion brown or yellow which is so deviated from black, originally favored color, or obtaining only products having considerably diminished values such that the blackened concentration at a light exposed section is low while the concentration at an unexposed portion is high.
European Patent No. 762,196, and JP-A-9-90,550 disclose that photosensitive silver halide particles used for the heat developable photosensitive materials contain periodic table VII-group or VIII-group (the seventh to tenth groups) metal ions or metal complex ions and that high contrast photographic characteristics can be obtained by containing hydrazine derivatives in the photosensitive materials. However, if the binder used in the coating liquid of the above water solvent and a nucleation agent such as hydrazine are concurrently used, a high contrast image can be obtained, but at the same time there raise problems such that fog and sensitivity deviations may likely occur. Particularly, there raise problems such that fog increase and sensitivity deviations during preservation are large.
Therefore, a technology is desired providing a heat developable photosensitive material capable of obtaining images with low fog, high Dmax (maximum density), and less fog increase and less sensitivity deviations during preservation, as well as having advantages in terms of environments and costs.
Accordingly, the first object to be accomplished by the invention is to provide a heat developable photosensitive material capable of obtaining images with low fog, high Dmax (maximum density), and less fog increase and less sensitivity deviations during preservation, particularly suitable for photomechanical processes as well as for scanners or image setters.
The second object of the invention to be solved is to provide a heat developable photosensitive material capable of coating with water with advantages in terms of environments and costs.